Field of the Invention
The invention relates to the field of electrotechnology and relates particularly to rotating electrical machines, therefore, for example, electric motors or generators.
Description of the Background Art
Particularly in automotive technology, i.e., in motor vehicles, electric motors are used increasingly in the form of actuating elements for assisting the driver or passengers (window lifter, sun roof drive, seat adjustment). In addition, electric motors are also used in support systems for the drive, for example, as an actuator for the gear unit and as fan motors and also in the actual vehicle drive itself. The electric motors employed there must satisfy more stringent requirements in that they are to be light to reduce vehicle weight and nevertheless have a high power density. In addition, a long service life and high reliability are required. Finally, particularly in the case of motors used near an internal combustion engine, an adequate operational reliability is required at temperatures of at least 120° C. or up to 180° C. as well
Basically brushed electric motors (commutator motors) or brushless electric motors can be used, whose motor is operated in a rotating field of a stator, whereby typically the stator is equipped with suitably controlled windings and the rotor is equipped with permanent magnets. Both the rotor and the stator are typically constructed by using lamination stacks, whereby the individual laminations are insulated from one another to reduce parasitic eddy currents, for example, by coated layers. Within the lamination stacks, so-called stator teeth with stator slots lying inbetween are provided in the stator, whereby the stator teeth support a field winding, which is disposed within the stator slots. The field winding is often controlled by an electronic circuit, which energizes individual winding parts according to a time regime and thereby generates a rotating field, whereby a pulse-width modulation technique is often used for controlling.
If high requirements for efficiency and power density are imposed on such motors, it becomes necessary for the rotor gap, i.e., the gap between the rotor and stator which also determines the distance between the permanent magnets, on the one hand, and the magnetic poles, on the other, to be designed to be as small as possible. Undesirable magnetic fields can arise in this case, for example, by self-induction, which can lead to demagnetization of the permanent magnets, particularly at higher operating temperatures. This consequently leads to a reduction of the performance and power density of the machine. To prevent this, magnetic materials or alloys with as high a coercive field strength as possible are usually used. Alloys with components or fractions of rare earth elements are often used to achieve the highest possible coercive field strength. Especially so-called heavy rare earth elements such as, for example, terbium and dysprosium have a high coercive field strength and therefore lend themselves to the stabilization of the permanent magnets. However, these materials are rare and cost-intensive.